IgG responses to salivary gland extract of Ixodes ricinus ticks vary inversely with resistance in naturally exposed sheep

Ticks and antibodies: May parasite density and tick evasion influence the outcomes following immunization protocols?

Ticks are a major concern to human health and livestock worldwide, being responsible for economic losses that go beyond billions of US dollars per year. This scenario instigates the development of vaccines against these ectoparasites, emphasized by the fact that the main method of controlling ticks still relies on the use of acaricides, what increases costs and may affect the environment as well as human and animal health. The first commercial vaccines against ectoparasites were produced against the tick Rhipicephalus microplus and their efficacy were based on antibodies. Many additional attempts have been conducted to produce protective immune responses against ticks by immunization with specific antigens and the antibody response has usually been the main target of evaluation. But some controversy still populates the roles possibly performed by humoral responses in tick-mammalian host relationships. This review focuses on the analysis of specific aspects concerning antibodies and ticks, especially the influence of parasite density and evasion/modulation. The immunization trials already described against R. microplus were also compiled and analyzed based on the characteristics of the molecules tested, protocols of immunization and tick challenge. Within these issues, it is discussed if or when antibody levels can be directly correlated with the development of tick resistance, and whether anti-tick protective immune responses generated by infestations may become ineffective under a different tick density. Also, higher titers of antibodies can be correlated with protection or susceptibility to tick infestations, what may be altered following continuous or repeated infestations and differ greatly comparing hosts with distinct genetic backgrounds. Regarding evasion, ticks present a sophisticated mechanism for dealing with antibodies, including Immunoglobulin Binding Proteins (IGBPs), that capture, transport and inject them back into the host, while keeping their properties within the parasite. The comparison of immunization protocols shows a total of 22 molecules already tested in cattle vaccination trials against R. microplus, with the predominance of concealed and dual antigens as well as marked differences in tick challenge schemes. The presence of an antibody evasion apparatus and variable levels of tick resistance when facing different densities of parasites are concerns that should be considered when testing vaccine candidates. Ultimately, more refinement may be necessary to effectively design a cocktail vaccine with tick molecules, which may be needed to be altered and combined in non-competing immune contexts to be universally secure and protective.

Ineffective humoral anti-tick IgY-response in birds: reaction against pathogen constituents?

Background: Variation in parasite burdens among hosts is typically related to differences in adaptive immunity. Comprehension of underlying mechanisms is hence necessary to gain better insights into endemic transmission cycles. Here we investigate whether wild songbirds that have never been exposed to ticks develop adaptive humoral immunity against endemic Ixodes ricinus ticks. Methods: Blue tits were exposed three times in succession to wild Ixodes ricinus ticks. For each infestation, serum samples were obtained. An enzyme-linked immunosorbent assay was developed, using tick salivary antigens, in order to quantify the bird's IgY response against ticks. In addition, at every sampling occasion the birds' body weight (corrected for body size) and haematocrit level was determined. Results: Individual IgY levels against the ticks' salivary proteins increased over three consecutive tick infestations, and large among-individual variation was observed. The responses were specifically directed against I. ricinus; cross-reactivity against the congeneric tree-hole tick Ixodes arboricola was negligibly low. IgY responses did not impinge on tick feeding success (engorgement weight and attachment success). Yet, those birds with the highest immune responses were more capable to reduce the acute harm (blood depletions) by compensating erythrocyte loss. Furthermore, at the end of the experiment, these birds had gained more body weight than birds with lower IgY levels. Conclusions: Latter observations can be considered as an effect of host quality and/or tolerance mechanisms. Birds anticipate the (future) costs of the activation of the immune system by ticks and/or ongoing tick-borne pathogen infections. Furthermore, although unsuccessful against tick feeding, the IgY responses may indirectly protect birds against tick-borne disease by acting against salivary protein secretions on which pathogens rely for transmission.

Possible Effects of Climate Change on Ixodid Ticks and the Pathogens They Transmit: Predictions and Observations

The global climate has been changing over the last century due to greenhouse gas emissions and will continue to change over this century, accelerating without effective global efforts to reduce emissions. Ticks and tick-borne diseases (TTBDs) are inherently climate-sensitive due to the sensitivity of tick lifecycles to climate. Key direct climate and weather sensitivities include survival of individual ticks, and the duration of development and host-seeking activity of ticks. These sensitivities mean that in some regions a warming climate may increase tick survival, shorten life-cycles and lengthen the duration of tick activity seasons. Indirect effects of climate change on host communities may, with changes in tick abundance, facilitate enhanced transmission of tick-borne pathogens. High temperatures, and extreme weather events (heat, cold, and flooding) are anticipated with climate change, and these may reduce tick survival and pathogen transmission in some locations. Studies of the possible effects of climate change on TTBDs to date generally project poleward range expansion of geographical ranges (with possible contraction of ranges away from the increasingly hot tropics), upslope elevational range spread in mountainous regions, and increased abundance of ticks in many current endemic regions. However, relatively few studies, using long-term (multi-decade) observations, provide evidence of recent range changes of tick populations that could be attributed to recent climate change. Further integrated 'One Health' observational and modeling studies are needed to detect changes in TTBD occurrence, attribute them to climate change, and to develop predictive models of public- and animal-health needs to plan for TTBD emergence.

A Natural Cattle Immune Response Against Horn Fly (Diptera: Muscidae) Salivary Antigens May Regulate Parasite Blood Intake

The horn fly, Haematobia irritans (L.), is a blood-sucking ectoparasite that is responsible for sizeable economic losses in livestock. The salivary gland products facilitate blood intake. Taking advantage of the identification of novel H. irritans salivary antigens (Hematobin, HTB and Irritans 5, IT5), we investigated the parasite loads, H. irritans blood intake, and antibody response of naturally infected bovines during the fly season. Fly loads and fly hemoglobin content fluctuated during the trial. Each time horn fly loads exceeded 200 flies per cattle, a reduction in horn fly blood intake was observed three weeks later. All of the cattle elicited an antibody response against HTB and IT5 that declined once the fly season was over. Cattle anti-IT5 titers were positively correlated with parasite loads and negatively correlated with fly blood intake. These results suggest that the natural changes in the H. irritans blood intake observed in this study were associated with a natural host response against horn fly salivary antigens.

Gastrointestinal helminths may affect host susceptibility to anthrax through seasonal immune trade-offs

Background

Most vertebrates experience coinfections, and many pathogen-pathogen interactions occur indirectly through the host immune system. These interactions are particularly strong in mixed micro-macroparasite infections because of immunomodulatory effects of helminth parasites. While these trade-offs have been examined extensively in laboratory animals, few studies have examined them in natural systems. Additionally, many wildlife pathogens fluctuate seasonally, at least partly due to seasonal host immune changes. We therefore examined seasonality of immune resource allocation, pathogen abundance and exposure, and interactions between infections and immunity in plains zebra (Equus quagga) in Etosha National Park (ENP), Namibia, a system with strongly seasonal patterns of gastrointestinal (GI) helminth infection intensity and concurrent anthrax outbreaks. Both pathogens are environmentally transmitted, and helminth seasonality is driven by environmental pressures on free living life stages. The reasons behind anthrax seasonality are currently not understood, though anthrax is less likely directly driven by environmental factors.

Results

We measured a complex, interacting set of variables and found evidence that GI helminth infection intensities, eosinophil counts, IgE and IgGb antibody titers, and possibly IL-4 cytokine signaling were increased in wetter seasons, and that ectoparasite infestations and possibly IFN-γ cytokine signaling were increased in drier seasons. Monocyte counts and anti-anthrax antibody titers were negatively associated with wet season eosinophilia, and monocytes were negatively correlated with IgGb and IgE titers. Taken together, this supports the hypothesis that ENP wet seasons are characterized by immune resource allocation toward Th-2 type responses, while Th1-type immunity may prevail in drier seasons, and that hosts may experience Th1-Th2 trade-offs. We found evidence that this Th2-type resource allocation is likely driven by GI parasite infections, and that these trade-offs may render hosts less capable of concurrently mounting effective Th1-type immune responses against anthrax.

Conclusions

This study is one of the first to examine laboratory-demonstrated Th1-Th2 trade-offs in a natural system. It provides evidence that seasonally bound pathogens may affect, through immunology, transmission dynamics of pathogens that might otherwise not be seasonally distributed. It suggests that, by manipulating the internal host ecosystem, GI parasites may influence the external ecosystem by affecting the dynamics of another environmentally transmitted pathogen.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-014-0027-3) contains supplementary material, which is available to authorized users.

Ticks and associated diseases: a retrospective review

This is a retrospective review of contributions to the understanding of ticks and associated diseases published in Medical and Veterinary Entomology since its first issue. It highlights the large and significant changes in the style and conduct of this field over the last 25 years. The selected papers refer to disease-related categories of host immunity to ticks, population dynamics, pathogen transmission and tick control.

Characterization of guinea pig antibody responses to salivary proteins of Triatoma infestans for the development of a triatomine exposure marker

Background

Salivary proteins of Triatoma infestans elicit humoral immune responses in their vertebrate hosts. These immune responses indicate exposure to triatomines and thus can be a useful epidemiological tool to estimate triatomine infestation. In the present study, we analyzed antibody responses of guinea pigs to salivary antigens of different developmental stages of four T. infestans strains originating from domestic and/or peridomestic habitats in Argentina, Bolivia, Chile and Peru. We aimed to identify developmental stage- and strain-specific salivary antigens as potential markers of T. infestans exposure.

Methodology and Principal Findings

In SDS-PAGE analysis of salivary proteins of T. infestans the banding pattern differed between developmental stages and strains of triatomines. Phenograms constructed from the salivary profiles separated nymphal instars, especially the 5^th instar, from adults. To analyze the influence of stage- and strain-specific differences in T. infestans saliva on the antibody response of guinea pigs, twenty-one guinea pigs were exposed to 5^th instar nymphs and/or adults of different T. infestans strains. Western blot analyses using sera of exposed guinea pigs revealed stage- and strain-specific variations in the humoral response of animals. In total, 27 and 17 different salivary proteins reacted with guinea pig sera using IgG and IgM antibodies, respectively. Despite all variations of recognized salivary antigens, an antigen of 35 kDa reacted with sera of almost all challenged guinea pigs.

Conclusion

Salivary antigens are increasingly considered as an epidemiological tool to measure exposure to hematophagous arthropods, but developmental stage- and strain-specific variations in the saliva composition and the respective differences of immunogenicity are often neglected. Thus, the development of a triatomine exposure marker for surveillance studies after triatomine control campaigns requires detailed investigations. Our study resulted in the identification of a potential antigen as useful marker of T. infestans exposure.

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi, and currently affects approximately 8 million people in Latin American countries. Although vector control campaigns against the most effective Chagas disease vector, Triatoma infestans, have been highly successful, T. infestans is re-establishing in once-endemic regions. To monitor re-establishing triatomines, new epidemiological tools are needed. Antibody responses of hosts to triatomine salivary proteins represent a promising tool to detect biting bugs, and highly immunogenic salivary antigens may be used as markers of triatomine exposure. Therefore, we analyzed the antibody response of guinea pigs, common peridomestic hosts of T. infestans, to salivary proteins of nymphs and adults of four different T. infestans strains from Argentina, Bolivia, Chile and Peru. Developmental stage- and strain-specific proteins in the saliva of T. infestans influenced the antibody response of guinea pigs, and different salivary antigens were recognized by guinea pig sera. Despite the variations of immunogenic salivary antigens, a 35 kDa antigen was recognized by almost all guinea pig sera and this antigen may be a useful marker of T. infestans exposure.

Changing geographic ranges of ticks and tick-borne pathogens: drivers, mechanisms and consequences for pathogen diversity

The geographic ranges of ticks and tick-borne pathogens are changing due to global and local environmental (including climatic) changes. In this review we explore current knowledge of the drivers for changes in the ranges of ticks and tick-borne pathogen species and strains via effects on their basic reproduction number (R₀), and the mechanisms of dispersal that allow ticks and tick-borne pathogens to invade suitable environments. Using the expanding geographic distribution of the vectors and agent of Lyme disease as an example we then investigate what could be expected of the diversity of tick-borne pathogens during the process of range expansion, and compare this with what is currently being observed. Lastly we explore how historic population and range expansions and contractions could be reflected in the phylogeography of ticks and tick-borne pathogens seen in recent years, and conclude that combined study of currently changing tick and tick-borne pathogen ranges and diversity, with phylogeographic analysis, may help us better predict future patterns of invasion and diversity.

The natural history of Anaplasma phagocytophilum

Anaplasma phagocytophilum is the recently designated name replacing three species of granulocytic bacteria, Ehrlichia phagocytophila, Ehrlichia equi and the agent of human granulocytic ehrlichiosis, after the recent reorganization of the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales. Tick-borne fever (TBF), which is caused by the prototype of A. phagocytophilum, was first described in 1932 in Scotland. A similar disease caused by a related granulocytic agent was first described in horses in the USA in 1969; this was followed by the description of two distinct granulocytic agents causing similar diseases in dogs in the USA in 1971 and 1982. Until the discovery of human granulocytic anaplasmosis (HGA) in the USA in 1994, these organisms were thought to be distinct species of bacteria infecting specific domestic animals and free-living reservoirs. It is now widely accepted that the agents affecting different animal hosts are variants of the same Gram-negative obligatory intracellular bacterium, which is transmitted by hard ticks belonging to the Ixodes persulcatus complex. One of its fascinating features is that it infects and actively grows in neutrophils by employing an array of mechanisms to subvert their bactericidal activity. It is also able to survive within an apparently immune host by employing a complex mechanism of antigenic variation. Ruminants with TBF and humans with HGA develop severe febrile reaction, bacteraemia and leukopenia due to neutropenia, lymphocytopenia and thrombocytopenia within a week of exposure to a tick bite. Because of the severe haematological disorders lasting for several days and other adverse effects on the host's immune functions, infected animals and humans are more susceptible to other infections.

Identification of a functional Antigen5-related allergen in the saliva of a blood feeding insect, the tsetse fly

Our previous screening of a Glossina morsitans morsitans lamdagt11 salivary gland expression library with serum of a tsetse fly exposed rabbit identified a cDNA encoding Tsetse Antigen5 (TAg5, 28.9 kDa), a homologue of Antigen5 sting venom allergens. Recombinant TAg5 was produced in Sf9 cells in order to assess its immunogenic properties in humans. Plasma from a patient that previously exhibited anaphylactic reactions against tsetse fly bites contained circulating anti-TAg5 and anti-saliva IgEs. In a significant proportion of plasma samples of African individuals, TAg5 and saliva binding IgEs (respectively 56 and 65%) can be detected. Saliva, harvested from flies that were subjected to TAg5- specific RNA interference (RNAi), displayed significantly reduced IgE binding potential. Allergenic properties of TAg5 and tsetse fly saliva were further illustrated in immunized mice, using an immediate cutaneous hypersensitivity and passive cutaneous anaphylaxis assay. Collectively, TAg5 was illustrated to be a tsetse fly salivary allergen, demonstrating that Antigen5-related proteins are represented as functional allergens not only in stinging but also in blood feeding insects.

Comparative IgG recognition of tick extracts by sera of experimentally infested bovines

Enzyme-linked immunosorbent assay (ELISA) and Western blot were used to investigate the pattern of antibody responses of six bovines infested twelve times with Rhipicephalus (Boophilus) microplus (Canestrini, 1887) (Acari: Ixodidae) (six heavy infestations followed by six light infestations) against salivary gland, gut and larvae extracts. During heavy infestations, bovine IgG levels were shown to be higher, and a decrease in the number and weight of ticks that completed the parasitic cycle was observed. The pattern changed starting from the seventh infestation, showing a decrease in IgG levels. An initial increase followed by a significant decrease in the proportion of ticks that completed the parasitic cycle was also observed from the seventh infestation. The number of molecules recognized by Western blot was higher from sera collected following heavy infestations than after light infestations, although a great variation in the profiles detected could be seen when the bovines were compared. These results indicate that IgG responses to different tick antigens may not be generally associated with bovine resistance, and that infestation levels modulate the magnitude of humoral responses and possibly the immune mechanisms in the natural acquisition of tick resistance.

Anaplasma phagocytophilum in ruminants in Europe

The agent that causes tick-borne fever (TBF) in sheep was first described in 1940, 8 years after the disease was first recognized in Scotland. The same agent was soon shown to cause TBF in sheep and pasture fever in cattle in other parts of the UK, Scandinavia, and other parts of Europe. After the initial use of the name Rickettsia phagocytophila, the organism was given the name Cytoecetes phagocytophila to reflect its association with granulocytes and its morphological similarity with Cytoecetes microti. This name continued to be used by workers in the UK until the recent reclassification of the granulocytic ehrlichiae affecting ruminants, horses, and humans as variants of the same species, Anaplasma phagocytophilum. TBF and pasture fever are characterized by high fever, recurrent bacteremia, neutropenia, lymphocytopenia, thrombocytopenia, and general immunosuppression, resulting in more severe secondary infections such as tick pyemia, pneumonic pasteurellosis, listeriosis, and enterotoxemia. During the peak period of bacteremia as many as 90% of granulocytes may be infected. The agent is transmitted transtadially by the hard tick Ixodes ricinus, and possibly other ticks. After patent bacteremia, sheep, goats, and cattle become persistently infected "carriers," perhaps playing an important role in the maintenance of infection, in the flock/herd. Little is known about how efficiently ticks acquire and maintain infection in ruminant populations or whether "carrier" domestic ruminants play an important role as reservoirs of infection, but deer, other free-living ruminants, and wild rodents are also potential sources of infection. During the late 1990s serological evidence of infection of humans was demonstrated in several European countries, creating a renewed interest and increased awareness of the zoonotic potential of TBF variants. More recently, a few cases of human granulocytic anaplasmosis (HGA) have been reported in some European countries, but it remains to be established whether the variants causing HGA in Europe are genetically and biologically different from those causing TBF in ruminants. TBF is readily diagnosed by demonstrating intracytoplasmic inclusions in peripheral blood granulocytes or monocytes of febrile animals or by detecting specific DNA by polymerase chain reaction (PCR), and TBF variants of A. phagocytophilum can be cultivated in tick cell lines, but the differentiation of TBF variants from HGA variants awaits further investigations.

Molecular Diagnosis of Granulocytic Anaplasmosis and Infectious Cyclic Thrombocytopenia by PCR-RFLP

Anaplasma phagocytophilum (A. phagocytophilum, formerly Ehrlichia phagocytophila) is a tick-borne pathogen responsible for tick-borne fever in ruminants, equine granulocytic ehrlichiosis (EGE) in horses, canine granulocytic ehrlichiosis (CGE) in dogs, and for human granulocytic ehrlichiosis (HGE). Human cases have been registered in many countries with a broad range of symptoms and pathogenicity. This article focused on Sardinia as the prevalence in humans was almost seven times higher than in the rest of Italy. To evaluate the risk, blood samples were collected from dogs and horses on the island. Genomic DNA was extracted from the buffy coat and amplified by heminested polymerase chain reaction (PCR) using the groEL gene primers. The first PCR reaction amplified a 624-bp fragment for both A. phagocytophilum and A. platys while the second PCR reaction amplified 573-bp and 515-bp fragments for the above two pathogens, respectively. Six A. phagocytophilum samples were PCR positive (3 dogs and 3 horses) while another dog was A. platys PCR positive. A phylogenetic analysis was conducted with A. phagocytophilum sequences in GenBank from the United States, Slovenia, Switzerland, Germany, UK, Austria, and Czech Republic. Surprisingly, the related phylogenetic tree showed that the Sardinian isolates were closer to the American isolates, which were showing highest mortality rates than from the other two European lineages.

Detection by the polymerase chain reaction of Anaplasma phagocytophilum in tissues of persistently infected sheep

To investigate the reservoir tissues of the tick-borne bacterium Anaplasma phagocytophilum in persistently infected sheep, six 6-month-old lambs were infected with a field isolate of the bacterium and maintained under tick-free conditions. At one and two weeks post-infection, A. phagocytophilum was detected in the peripheral blood of all lambs by examining May-Grünwald Giemsa-stained blood smears for classical intra-neutrophil inclusions, and by an A. phagocytophilum-specific nested PCR. After euthanasia at 3 months post-inoculation, peripheral blood and numerous tissue samples were collected from each lamb. DNA extracted from these samples was then subjected to PCR. All blood samples were PCR-negative but three lambs had PCR-positive tissues including intestinal wall and lymph nodes, thymus, bone marrow, kidney and bladder wall. The widespread nature of PCR-positive tissues suggested that circulatory cells may form the reservoir cells for A. phagocytophilum infection in carrier sheep, rather than lymphoid tissues as in rodents. PCR-positive tissue and blood samples were strikingly fewer in the experimentally infected sheep than reported earlier in tick-exposed carrier sheep under field conditions. It seems possible that tick infestation amplifies A. phagocytophilum infections in carrier sheep to a degree that enables tick transmission to occur.

Tick saliva in anti-tick immunity and pathogen transmission

When feeding on vertebrate host ticks (ectoparasitic arthropods and potential vectors of bacterial, rickettsial, protozoal, and viral diseases) induce both innate and specific acquired host-immune reactions as part of anti-tick defenses. In a resistant host immune defense can lead to reduced tick viability, sometimes resulting in tick death. Tick responds to the host immune attack by secreting saliva containing pharmacologically active molecules and modulating host immune response. Tick saliva-effected immunomodulation at the attachment site facilitates both tick feeding and enhances the success of transmission of pathogens from tick into the host. On the other hand, host immunization with antigens from tick saliva can induce anti-tick resistance and is seen to be able to induce immunity against pathogens transmitted by ticks. Many pharmacological properties of saliva described in ticks are shared widely among other blood-feeding arthropods.

TICKCIDAL EFFECT OF MONOCLONAL ANTIBODIES AGAINST HEMOCYTES, OM21, IN AN ADULT FEMALE TICK, ORNITHODOROS MOUBATA (ACARI: ARGASIDAE)

In the present study, monoclonal antibodies (mAbs) against adult Ornithodoros moubata hemocytes were established. Afterward, artificial feeding was performed to assess the tickcidal effect of fetal bovine serum meal containing each mAb. As a result, Om21 showed the strongest tickcidal effect on adult female O. moubata. The reactivity of various tick cells and organs, including the hemocyte, midgut, trachea, ovary, fat body, and muscle, to Om21 was then examined by an indirect immunofluorescent antibody test and by immunoelectron microscopy. Om21 reacted with not only hemocytes but also with fat body cells, epidermis, cuticle of the trachea, connective tissue of the muscle, and the basement membrane of the midgut, trachea, fat body, oocyte, and epidermis. These results suggest that Om21 passing through the midgut epithelium induced a tickcidal effect on hemocytes or various organs. However, the target of Om21 could not be identified in the present study. The antihemocyte mAb produced in this study, Om21, may be useful for the immunological control of ticks.

Transmission of Anaplasma phagocytophilum to Ixodes ricinus ticks from sheep in the acute and post-acute phases of infection

A total of 60 sheep were exposed to Anaplasma phagocytophilum infection on an enclosed area of Ixodes ricinus-infested pasture in North Wales, United Kingdom, and rapidly acquired acute A. phagocytophilum infections detectable by PCR and blood smear examination. Of the ticks that had engorged in the previous instar on infected sheep, 52% of adult ticks and 28% of nymphs were PCR positive; a significant, 10-fold increase in prevalence compared to that of ticks that engorged on sheep preinfection was observed (P = 0.015). The likelihood that ticks were PCR positive, after feeding on the sheep and molting to the next instar, increased marginally with increasing numbers of infected neutrophils per milliliter of blood of their sheep host (P = 0.068) and increased significantly when they were collected from sheep carrying higher numbers of adult female ticks (P = 0.017), but increasing numbers of feeding nymphs had a significant negative effect on transmission (P = 0.049). The numbers of circulating neutrophils and of infected neutrophils also varied significantly with the numbers of ticks feeding on the sheep when the blood was collected. Our study suggests that ruminants are efficient reservoirs of A. phagocytophilum during the acute and post-acute phases of infection. The risk of ruminant-derived infections may, however, be strongly affected by variations in tick densities, which may influence transmission from acutely infected animals via effects on the numbers of infected cells in the blood and possibly by within-skin modulation of infection.